1. Technical Field
The present invention relates to a fuel return device for recovering surplus fuel of the fuel supplied to an internal combustion engine simultaneously from a plurality of fuel tanks, and returning same to the fuel tanks, and more particularly, to a fuel return device whereby the residual amounts of fuel in the respective fuel tanks can be made equal.
2. Background Art
Recently, a DME internal combustion engine which uses dimethyl ether (hereinafter, called “DME”) as an alternative fuel instead of gasoline or diesel oil, has become known.
A fuel supply system for a DME internal combustion engine (hereinafter, simply called “engine”) is described with reference to FIG. 3.
In this fuel supply system, two fuel tanks 31a and 31b filled with fuel (DME) are provided in the chassis of the vehicle (not illustrated). Pressure feed pumps 32a and 32b for supplying DME under pressure are provided respectively at the fuel tanks 31a and 31b. The fuel inside the fuel tanks 31a and 31b is supplied respectively by the pressure feed pumps 32a and 32b to a fuel pipe 33, and the pressure of the fuel is raised by a secondary pump 34. Thereupon, the fuel is supplied to a supply pump 35 provided in the engine E. A common rail 37 for accumulating pressurized fuel that is to be injected is connected to the supply pump 35, and a plurality of fuel injection valves (injectors) 36 for injecting fuel into the combustion chambers of the respective cylinders of the engine E, are connected to the common rail 37. The supply pump 35 raises the fuel to a pressure suitable for injecting into the combustion chamber, and supplies the fuel to the common rail 37.
The secondary pump 34 and the supply pump 35 are pressure control pumps which are capable of adjusting the output pressure, and they are connected respectively to fuel recovery pipes 38a and 38b which serve to return surplus fuel that has been discharged from the pumps 34 and 35, to the fuel tanks 31a and 31b. A fuel recovery pipe 38c for returning surplus fuel discharged from the common rail 37 to the fuel tanks 31a and 31b is connected to an intermediate point of the fuel recovery pipe 38b. A pressure control valve (PCV) 40 is provided in the fuel recovery pipe 38c and the pressure inside the common rail 37 is adjusted to the optimal pressure for fuel injection. The fuel recovery pipes 38a and 38b are joined to a single main fuel return pipe 39. Therefore, the surplus fuel from the secondary pump 34, the supply pump 35 and the common rail 37 is all recovered into the main fuel return pipe 39. Two subsidiary fuel return pipes 44a and 44b branch off from the downstream end of the main fuel return pipe 39, and these subsidiary fuel return pipes 44a and 44b are connected respectively to the fuel tanks 31a and 31b. The surplus fuel recovered into the main fuel return pipe 39 is divided between these subsidiary fuel return pipes 44a and 44b and returned to the respective fuel tanks 31a and 31b via the same. In the diagram, numeral 43 denotes a fuel cooler and numeral 45 denotes a fuel shut-off valve.
Here, the amount of heat generated by the DME is approximately 28.8 MJ/kg, which is relatively low compared to the figure of approximately 42.7 MJ/kg in the case of diesel oil. Therefore, it is necessary to increase the amount of fuel injected into the engine in order to obtain output performance equivalent to that of a diesel engine running on diesel oil. Accordingly, the amount of fuel consumed becomes very large, and in order to allow the vehicle to travel a long distance with one filling of the full tank, it is necessary to increase the capacity of the fuel tank. Therefore, a plurality of fuel tanks 31a and 31b may be provided in the vehicle, as illustrated in FIG. 3. Furthermore, in some cases, the fuel tanks are divided into a plurality of separate bodies, due to layout criteria.
In a vehicle that is fitted with a plurality of fuel tanks, if the fuel in the respective fuel tanks is used in a sequential fashion such that the supply of fuel switches from one fuel tank to the next fuel tank when one fuel tank has become empty, then it is necessary to provide a large-capacity pressure feed pump, respectively, at each fuel tank due to the fact that a DME requires a large amount of fuel to be supplied to the fuel injection valves, as described previously. This leads to increased costs. Moreover, providing a large pressure feed pump inside each fuel tank may also be problematic in terms of space.
Therefore, in the fuel supply system illustrated in FIG. 3, fuel is supplied to the fuel injection valves 36 simultaneously from each of the fuel tanks 31a and 31b. More specifically, the pressure feed pumps 32a and 32b of the plurality of fuel tanks 31a and 31b are actuated simultaneously. In so doing, it becomes possible to use pressure feed pumps 32a and 32b of relatively smaller capacity in the respective fuel tanks 31a and 31b. 
If fuel is supplied simultaneously from a plurality of fuel tanks 31a and 31b, then the surplus fuel from the secondary pump 34, the supply pump 35 and the common rail 37 is distributed between the respective fuel tanks 31a and 31b. 
However, there may be cases where the temperatures and pressures inside the fuel tanks 31a and 31b are not equal, due to the use conditions of the vehicle, the driving conditions, or the like. Consequently, the amount of fuel branching off from the main fuel return pipe 39 into the respective subsidiary fuel return pipes 44a and 44b and flowing from these into the respective fuel tanks 31a and 31b is not equal, and hence a problem arises in that the rate of reduction (the residual amount) of the fuel inside the fuel tanks 31a and 31b becomes unequal. In a worst case scenario, one of the fuel tanks 31a or 31b may become empty before the other fuel tank and hence the corresponding pressure feed pump 32a or 32b may be driven without fuel, thereby causing damage to the pump.